Muscular dystrophy refers to a group of genetically determined myopathies characterized by progressive atrophy or degeneration of increasing numbers of individual muscle cells. The structural changes observed histologically are essentially the same in the various types of muscular dystrophies. This may, perhaps, suggest a common etiology. However, the distribution of the affected muscles is quite distinctive. This, along with the mode of inheritance, forms the basis of the classification of these diseases. The muscular dystrophies are traditionally subdivided by the patterns of initial muscle involvement, which in turn correlates fairly well with the type of genetic transmission. The three major forms of muscular dystrophy are as follows: 1) Duchenne's Muscular Dystrophy which affects most skeletal muscle groups and is transmitted by an X-linked recessive gene; 2) Limb Girdle Muscular Dystrophy, affecting principally the pelvic and shoulder girdle muscles and is transmitted by an autosomal recessive gene; and 3) Facioscapulohumeral Muscular Dystrophy, involves the muscles of the face and shoulder girdle and is transmitted by an autosomal dominant gene.
Recently, the defective gene responsible for Duchenne's Muscular Dystrophy (DMD) has been located on the X-chromosome. The DMD gene encodes for a large molecular weight protein product, called dystrophin. This protein is localized to the sarcolemmal membrane of normal skeletal muscle, but is absent from the skeletal muscle of people with DMD, as well as dogs and mice with dystrophic muscle. A more benign form of this X-linked recessive disease is Becker's Muscular Dystrophy which is caused by an abnormal DMD gene which encodes an abnormal dystrophin protein. The exact function of dystrophin and the reasons why its absence or abnormal structure results in necrosis of dystrophic muscle fibers has not been determined. However, the amino acid sequence of dystrophin suggests that it is a membrane cytoskeletal protein.
The present technology for initial detection and diagnosis of Duchenne's or Becker's Muscular Dystrophy relies on the use of an immunological probe to identify the presence of dystrophin, the absence of dystrophin, or the abnormal molecular weight or content of dystrophin in human muscle biopsies. It is not uncommon for genetic diseases to involve the loss or abnormal synthesis of more than one component or protein. In the case of muscular dystrophy, proteins other than dystrophin may be involved which are translated from genes located on different chromosomes (X chromosomes and/or autosomal chromosomes), resulting in the different forms of muscular dystrophy. The identification of other potential proteins involved in muscular dystrophy and methods of quantifying these proteins would be immensely useful to clinicians for confirming diagnosis of Duchenne's and Becker's muscular dystrophy, as well as, perhaps providing an initial diagnosis of other forms of muscular dystrophy. In addition, knowledge of the function of these proteins may lead to methods of predicting prognosis of disease progression and perhaps therapeutic treatments for patients with muscular dystrophy in all of its various forms.